Bone screw

ABSTRACT

A bone screw is provided having first and second threaded portions of different diameter separated by a middle portion that is unthreaded so that the bone screw may be inserted into a pair of adjacent bone fragments and used to compress the fragments together by driving the screw into a hole drilled in the fragments. An installation tool is also disclosed for threadably engaging one end of the bone screw and is used to initially drive the screw into the bone hole. Thereafter, a screwdriver may be inserted through the tool and into engagement with a recess in the bone screw and may be used to drive the screw fully into the bone and to separate the bone screw from the installation tool. A method of installation is further provided.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of the U.S. national phasedesignation of co-pending international application PCT/CH01/00698 toFrenk, filed Dec. 4, 2001, the entirety of which application is herebyincorporated by reference thereto.

FIELD OF THE INVENTION

The invention relates to a bone screw for connecting two bone fragments,to a device for implanting such a bone screws and to a method forsetting, compressing and/or fixing bone fragments.

BACKGROUND OF THE INVENTION

Bone screws are used in various ways in osteosynthesis, for example, forsetting bone fragments, as compression screws or for fixing bonefragments.

A bone screw with two axially terminal threaded segments and a middlethreadless segment is known from U.S. Pat. No. 5,019,079 to Ross. Thediameter of the middle segment corresponds essentially to the externaldiameter of the external thread at the distal threaded segment, but islarger than the core diameter of the external thread at the proximalthreaded segment, so that the middle segment can be used for laterallystabilizing the two bone fragments of the fracture. It is a disadvantageof this construction of bone screws that the two external threads havedifferent pitches, so that the different steps for the implantation, thesetting the bone fragments, the compression of the bone fragments andthe recessing of the screw head cannot be carried out separately fromone another.

The invention is to provide a remedy to this problem. It is an object ofthe invention to create a bone screw, which enables the bone fragmentsto be set and compressed and the screw head to be recessed separatelyduring the implantation.

SUMMARY OF THE INVENTION

Pursuant to the invention, this objective is accomplished with a bonescrew, and with a device for implanting such a bone screw, as well aswith a method for setting, compressing and/or fixing bone fragments

The inventive bone screw comprises essentially two threaded segments,which are disposed coaxially with the longitudinal axis and terminallyat the bone screw, the pitches S_(V) and S_(H) of the front and rearsegments respectively being identical. After these two bone fragmentshave been set and compressed, wherein only the front threaded segment isscrewed into the distal bone fragment while the rear threaded segment isscrewed, for example, into an implantation instrument and not yet intothe proximal bone fragment, the bone screw can be screwed further intothe bone fragments, until the rear threaded segment also is recessedcompletely in the proximal bone fragment. This can be accomplishedwithout at the same time changing the position of the bone fragmentsrelative to one another and without changing the compression of the twobone fragments. The two threaded segments are constructed so that theexternal diameter of the front threaded segment is smaller than the corediameter of the external thread at the rear threaded segment.

The advantages of the inventive bone screw and the inventive device arethat due to the pitch of the external thread at the front threadedsegment and at the rear threaded segment being the same, the steps ofsetting the bone fragments, compressing the bone fragments, andrecessing the head of the screw can be carried out separately and in acontrolled manner.

Because the rear threaded segment is configured with a core diameter,which is larger than the external diameter of the front threadedsegment, interaction of the rear threaded segment with the threadalready cut in the bone fragments for the front threaded segment can beavoided.

Preferably, the external threads at the front and rear threaded segmentsare self-cutting threads.

A preferred embodiment of the inventive bone screw, includes, betweenthe two threaded segments, a middle, threadless segment, which has anexternal diameter, which is smaller than or equal to the core diameterof the external thread at the front threaded segment. With that, thefront threaded segment can be screwed completely into the distal bonefragment and the borehole in the proximal bone fragment does not have tobe enlarged relative to the borehole in the distal bone fragment forsetting and compressing the bone fragments. Compared to embodiments ofknown bone screws, the front threaded segment of which directly adjoinsthe rear threaded segment in the axial direction and for which theborehole in the proximal bone fragment would have to be enlarged so thatthe front threaded segment can be screwed only into the distal bonefragment, a higher stability of the connection between the bone screwand the proximal bone fragment can furthermore be attained with thepresent device.

The inventive device serves for setting, compressing and fixing bonefragments by means of a bone screw and includes a surgical implantationinstrument, which has a central borehole through which a screwdriver canbe passed, extending coaxially through the implantation instrument.Furthermore, the central borehole is expanded from the front end of theimplantation instrument up to a depth T, so that a shoulder is formed atthe depth T. In the expanded part of the central borehole, there is aninternal thread, which is complementary to the external thread of therear threaded segment of the bone screw, so that the rear threadedsegment of the bone screw can be screwed into the central borehole up toa depth T. The depth T is selected so that T≧L, where L is the length ofthe rear threaded segment of the bone screw. With that, it can beachieved that the rear, threaded segment of the bone screw can bescrewed completely into the central borehole of the implantationinstrument.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and further developments of the invention are explained ineven greater detail in the following by means of the partiallydiagrammatic representations of several examples. In the drawings,

FIG. 1 shows a cross-sectional view of an embodiment of the inventivebone screws; and

FIG. 2 shows a cross-sectional view of the embodiment shown in FIG. 1,an implantation instruments, and a screwdriver.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the inventive bone screw 1 is shown in FIG. 1.This bone screw 1 includes a rear threaded segment 7 with an externalthread 9, which has a core diameter D_(KH), an external diameter D_(HS)and a pitch S_(H), a middle, threadless segment 6 with an externaldiameter D_(MS), which adjoins the rear threaded segment 7 coaxiallywith the longitudinal axis 2, and a front threaded segment 5 with anexternal thread 8, which has a core diameter D_(KV), an externaldiameter D_(VS) and a pitch S_(V). The two threaded segments 5, 7 havedifferent diameters, that is, the core diameter D_(KH) of the rearthreaded segment 7 is larger than or equal to the external diameterD_(VS) of the front threaded segment 5. However, the pitches of the twoexternal threads 8, 9 are identical. The external diameter D_(MS) of themiddle segment 6 is smaller than or equal to the core diameter D_(KV) ofthe front threaded segment 5. Moreover, at the front end 3 of the bonescrew 1 and at the transition between the rear threaded segment 7 andthe middle segment 6, several indentations 23, distributed over theperiphery of the two threaded segments 5, 7 and aligned axially, aredisposed with cutting edges 12 essentially parallel to the longitudinalaxis 2, so that these two external threads 8, 9 are constructed asself-cutting threads. At the rear end of 4 of the bone screw 1, means 11for accommodating a screwdriver, for example, a hexagonal recess, Torxor Phillips, are disposed coaxially. Moreover, the bone screw 1 isequipped with a central borehole 10, which extends from the front end 3up to the rear end 4 and serves, for example, for accommodating aguiding wire (not shown).

In FIG. 2, the inventive device is shown together with a bone screw 1,the rear threaded segment 7 is screwed completely into the implantationinstrument 15 and the front threaded segment 5 of which is screwedcompletely into the distal bone fragment 14. The implantation instrument15 includes a continuous central borehole 17, which is expanded from thefront end 18 up to a depth T and, in expanded part 24, has an internalthread 20, which is complementary to the external thread 9. At the depthT, between the expanded part 24 of the central borehole 17 and thenarrower part 25 of the central borehole 17, there is a shoulder 22,against which the rear end 4 of the bone screw 1 rests when the rearthreaded segment 7 is screwed completely into the implantationinstrument 15. A screwdriver 16 can be passed through the narrower part25 of the central borehole 17 from the rear end 19 of the implantationinstrument 15, so that the screwdriver 16 can be introduced into themeans 11, which are disposed at the rear end 4 of the bone screw 1 foraccommodating a screwdriver and the bone screw 1 can be rotated by meansof the screwdriver 16 relative to the implantation instrument 15.

For setting, compressing and fixing the two bone fragments 13, 14, aborehole 21, passing through the proximal bone fragments 13 and into thedistal bone fragment 14, is produced. The diameter of the borehole 21corresponds to the core diameter D_(KV) (FIG. 1) of the external thread8 at the front threaded segment 5 of the bone screw 1.

At the start of the implantation process, the rear, threaded segment 7of the bone screw 1 is screwed completely and up to a depth T in theinternal thread 20 into the central borehole of the implantationinstrument 15. By rotating the implantation instrument 15 about thelongitudinal axis 2, the bone screw is then screwed into the pre-drilledboreholes 21 in the two bone fragments 13, 14. Since the rear threadedsegment 7 of the bone screw 1 is taken up completely in the implantationinstrument 15, the external thread 9 of the rear threaded segment 7cannot engage the proximal bone fragments 13, so that, as theimplantation instrument 15 is rotated, only the front threaded segment 5of the bone screw 1 can be screwed into the distal bone fragment 14. Inthis phase, the front end 18 of the implantation instrument 15 assumesthe task of a screw head, so that, after the bone screw 1 has beenbrought into the two bone fragments 13, 14 far enough that the front end18 of the implantation instrument 15 lies against the proximal bonefragment 14, the two bone fragments 13, 14 are moved towards one anotherby rotating the implantation instrument 15 further. As soon as the twobone fragments 13, 14 are in contact with one another, compression ofthe two bone fragments 13, 14 commences. As soon as the desiredcompression of the two bone fragments 13, 14 has been reached byrotating the implantation instrument 15 further, the screwdriver 16 isinserted through the central borehole 17 in the implantation instrument15 into the means 11 for accommodating the screwdriver and the bonescrew 1 is rotated further with the screwdriver 16, so that, while theimplantation instrument 15 is held in place, the bone screw 1 is screwedout of the internal thread 20 at the front end 18 of the implantationinstrument 15 and the rear threaded segment 7 is screwed into theproximal bone fragments 13, until the rear threaded segment 7 is broughtcompletely beneath the surface of the proximal bone fragment 13. Sincethe two bone fragments 13, 14 are not moved relative to one anotherduring this last process, the compression is unchanged after the rear,threaded segment 7 is driven into the proximal bone fragment 13.

Preferably, the bone screw 1 is used where a screw head would interfere,for example, for fractures in the vicinity of a joint, forintraarticular fixation such as scaphoid fractures, for small fragmentsand for fixations in the vicinity of sinews, nerves and vessels.

While the invention has been shown and described herein with referenceto particular embodiments, it is to be understood that the variousadditions, substitutions, or modifications of form, structure,arrangement, proportions, materials, and components and otherwise, usedin the practice and which are particularly adapted to specificenvironments and operative requirements, may be made to the describedembodiments without departing from the spirit and scope of the presentinvention. Accordingly, it should be understood that the embodimentsdisclosed herein are merely illustrative of the principles of theinvention, and that various modifications may be made by those skilledin the art which will embody the principles of the invention and fallwithin the spirit and the scope thereof.

1. A bone screw for connecting a plurality of bone fragments comprising:a shaft extending along a longitudinal axis, the shaft including a firstend having a first threaded portion with a first core diameter, a firstexternal diameter and a first pitch, the second end having a secondthreaded portion with a second core diameter, a second external diameterand a second pitch, the second end further having a tool engagingsurface, wherein the first external diameter is no greater than thesecond core diameter, and the first and second pitches are substantiallyequal, and a non-threaded middle segment between the first and secondthreaded portions, an outer diameter of the middle segment being smallerthan the first core diameter, and wherein the shaft includes acannulation extending therethrough disposed coaxial with the screwlongitudinal axis.
 2. The screw of claim 1, wherein the tool engagingsurface comprises a recess configured to be engaged by a driving end ofa screwdriver for imparting rotational motion to the bone screw.
 3. Thescrew of claim 2, wherein the recess comprises a hexagonal recess, aTorx recess or Phillips recess.
 4. The screw of claim 3, wherein thefirst threaded portion has self-cutting threads for cutting bone.
 5. Thescrew of claim 4, wherein the first threaded portion comprises at leastone surface disposed substantially parallel to the screw longitudinalaxis configured for cutting bone.
 6. The screw of claim 4, wherein thesecond threaded portion has self-cutting threads for cutting bone. 7.The screw of claim 6, wherein the second threaded portion comprises atleast one surface disposed substantially parallel to the screwlongitudinal axis configured for cutting bone.
 8. The screw of claim 1,wherein the cannulation is configured to receive a guide wire forguiding the screw into the bone.